I believe it's part of your fire selector. Should be a bunch of wires that connect the selector plate to the battery and motor. (Someone correct me if I'm wrong)

Kinda.

The switch assembly is the part of the wiring harness that contains the trigger contacts. Two pieces of metal that are pushed together when you pull the trigger, completing the circuit of the gun. When these are pushed together, it creates a path for power to flow from the battery to the motor.

Sorry if its a shitty explanation, Its late

EDIT
Okay picture, When you pull your trigger. It moves the part on the left, into the part on the right. When the metal on these two parts touches, the circuit is complete, and your AEG will fire.